The prior art has recognized two distinct processes for the removal of sulfur oxides from gases: scrubbing with lime or neutralization with sodium alkalis. The use of a lime slurry for direct dry scrubbing in a quench reactor results in the vaporization of the slurry water and the formation of dry salt reaction products. The effluent stream leaving the quench reactor contains the reaction products (salts of acid gases), unreacted reagent, particulates and unreacted gases. This scrubbing is typically conducted at low stack temperatures. The efficiency of lime slurry neutralization utilizing economic levels of reagent is relatively low, typically on the order of 70 to 80% SO.sub.x capture and neutralization. It has been determined that on an experimental basis the capture and neutralization of SO.sub.x can be increased when a limestone slurry is `doped` with an alkali salt (see Air Pollution Control Association, January 1983, Vol. 33, No. 1, "Activated Wet-Dry Scrubbing of SO.sub.2 ", Karlsson et al). The neutralization reaction continues after the effluent gas leaves the quench reactor.
The efficiency of SO.sub.x capture and neutralization by sodium alkalis, is somewhat higher than that achieved with lime slurry, typically on the order of 85 to 95%. However, when using sodium alkalis, the products are water-soluble salts which present a major disposal problem.
In these prior art processes a baghouse is commonly employed to remove the dry products formed in the quench reactor. The dry products are retained as filter cake and the filtered gases are discharged to the atmosphere. The filters are periodically cleaned by shaking, reverse air flow or by pulsed air jets. The adhesive-like characteristics of the dry product (non free flowing) necessitates that the filters be cleaned frequently, and if this is not done, the dry products blind the filter fabric, causing an unacceptably high pressure drop across the baghouse.
The present invention stems from the discovery that the efficiency of SO.sub.x removal in a lime slurry neutralization process can be increased to over 90% by the interposition of a dry venturi between the quench reactor and the baghouse without resorting to a reduction of temperature close to the dew point. The calcium based reagent used in the quench reactor is doped. The dry venturi allows the filter cake in the baghouse to be accumulated to a sufficient depth such that the baghouse functions as a secondary (fixed bed) reactor.
The filter cake contains unreacted reagent due to the normal excess reagent fed to the system. This unreacted reagent neutralizes the residual acid gases flowing through the baghouse.
The filter cake (dry reaction products) accumulated in the baghouse is substantially non-tacky. Thus, the cake buildup in the baghouse may be severalfold over that which can be tolerated in prior art processes. This in turn makes it possible to extend the duration between cleaning cycles to as much as 50 times that of conventional processes. This increase in cycle time permits the accumulation of a fixed bed cake in the baghouse.
Broadly, the invention comprises a method for the removal of sulfur oxides from a gaseous stream with greater efficiencies than conventional processes. The method avoids the necessity for dangerous conditions of operating a baghouse at a temperature close to the dew point. The gaseous stream containing sulfur oxides is introduced into a quench reactor and is contacted with a calcium-based reagent containing between about 3 to 30% by weight of a hygroscopic alkaline metal cation salt, based on the total weight percent of the calcium compound in the reagent. Such salts include CaCl.sub.2, MgCl.sub.2, FeCl.sub.3, MnCl.sub.2, ZnCl.sub.2, CrCl.sub.2, CdCl.sub.2, FeO.sub.2. The effluent stream from the reactor contains particulates, salts of acid gases containing unreacted reagent, and residual acid gases. The effluent stream flows through a dry venturi. A separate stream containing sorptive material and/or fly ash from the baghouse product stream is introduced into the dry venturi to contact the effluent stream to remove submicron particulates therefrom. The effluent stream from the dry venturi flows into a separator where the solids in the stream are collected and form a filter cake. The acid gases in the stream have interfacial contact with the cake and a substantial portion of the SO.sub.x entering the separator is neutralized. The total SO.sub.x removed from the gaseous stream is at least 90%.